The spread of FTTH (Fiber To The Home) is progressing globally due to an increasing need for high-speed access services. Most FTTH services are provided by an economically excellent PON (Passive Optical Network) system, in which a single storage station-side device (OSU: Optical Subscriber Unit) stores a plurality of subscriber-side devices (ONU: Optical Network Unit) by time division multiplexing (TDM).
In an upstream communication by a TDM-PON system, the system bandwidth is shared between the ONUs based on a dynamic bandwidth allocation calculation in the OSU, and each ONU intermittently transmits an optical signal only within a transmission permission time period notified by the OSU, thereby preventing collision between optical signals.
The current main systems are GE-PON (Gigabit Ethernet (registered trademark) PON) and G-PON (Gigabit-capable PON), which have gigabit-level transmission speeds. In addition to the progress of video distribution services, the emergence of applications that upload/download large-capacity files requires further increases in the capacity of PON systems.
However, in the TDM-PON system described above, since the system bandwidth is expanded by increasing the line rate, the reception characteristics are greatly deteriorated due to the effects of higher speed and wavelength dispersion, and further, the economy of the burst transceiver becomes a problem, thereby making it difficult to increase the capacity to more than 10 gigabytes.
Application of a wavelength division multiplexing (WDM) technique is being investigated for increasing the capacity to more than 10 gigabytes. FIG. 1 is an example of a WDM/TDM-PON system in which a WDM technique is combined with a TDM-PON system related to the present invention.
The WDM/TDM-PON system shown in FIG. 1 includes OSUs 10 #1 to #M and a plurality of ONUs 93. The OSUs 10 #1 to #M respectively communicate with the plurality of ONUs 93 using a wavelength set of λU_1,D_1 to λU_M,D_M. Here, λU_1,D_1 indicates a combination of an upstream signal wavelength λU_1 and a downstream signal wavelength λD_1.
The OSU in the present specification refers to an OLT CT (Channel Termination) prescribed by the International Standard ITU-T (Telecommunication standardization of International Telecommunication Union) G.989.3 (40-Gigabit-capable passive optical networks (NG-PON 2): Transmission Convergence Layer Specification). Furthermore, the WDM/TDM-PON in the present specification refers to a TWDM-PON (Time and Wavelength Division Multiplexing-PON) prescribed by the International Standard ITU-T G.989 series.
Each ONU 93 is fixedly assigned a downstream wavelength and an upstream wavelength according to the terminal of a wavelength routing unit 94-1 to which it is connected. Temporal signal overlap among all ONUs 93 is permitted for #1 to #M, that is to say, up to the number of OSUs 10. Consequently, by adding an OSU 10, the system bandwidth can be expanded without increasing the line rate per wavelength.
Among the terminals of the wavelength routing unit 94-1, each ONU 93 connected to the same terminal on the ONU 93 side and connected to an optical fiber transmission line 96 is logically connected to the same OSU 10, and share an upstream bandwidth and a downstream bandwidth.
For example, the ONUs 93 #2-1 to #2-K are logically connected to the OSU 10 #2. Here, the logical connection between each ONU 93 and the OSU 10 is constant, and it is not possible to distribute traffic load among different OSUs 10 #1 to #M according to the state of traffic load of each OSU 10.
On the other hand, as shown in FIG. 2, proposed is a wavelength tunable WDM/TDM-PON system whose optical transmitter and optical receiver mounted on the ONU 93 are equipped with a wavelength tuning function (for example, refer to Non-Patent Document 1).
The ONU 93 includes a wavelength tunable light transmission unit 31, a wavelength tunable light reception unit 32 having a light receiving unit 321 and a wavelength tunable filter 322, and a wavelength multiplexing and demultiplexing unit 33.
The wavelength multiplexing and demultiplexing unit 33 outputs an optical signal to the wavelength tunable light reception unit 32, and receives an optical signal from the wavelength tunable light transmission unit 31. The wavelength tunable filter 322 inputs to the light receiving unit 321, a signal of a wavelength selected by performing filtering of the optical signal output by the wavelength multiplexing and demultiplexing unit 33. The optical multiplexing and demultiplexing units 92-1 and 92-2 and the optical fiber transmission line 96 connect an OLT 91 and the ONU 93.
In the configuration of Non-Patent Document 1, it is possible to individually change the logical connection destination OSU 10 of each ONU 93 by switching the transmission/reception wavelength in the ONU 93. As a result of using this function, when there is an OSU 10 in a high-load state, the logical connection between the ONU 93 and the OSU 10 is changed so that the traffic load is dispersed to an OSU 10 in a low-load state, and it is possible to prevent a deterioration in the communication quality of the OSU 10 in the high-load state.
Furthermore, when a high-load state of an OSU 10 regularly occurs, in the WDM/TDM-PON system of FIG. 1, it is necessary to add system bandwidth in order to ensure a fixed communication quality. On the other hand, in the wavelength tunable WDM/TDM-PON system of FIG. 2, it is possible to ensure a fixed communication quality by effectively utilizing the bandwidth of the entire system by distributing the traffic load among the OSUs 10, and capital investments for expanding the system bandwidth can be suppressed.
FIG. 3 shows an example of wavelength assignment in the wavelength tunable WDM/TDM-PON system related to the present invention. In FIG. 3, a bandwidth of 6 Gbit/s total is used for the OSU 10 #1, which includes a downstream signal #1 (1 Gbit/s) and a downstream signal #2 (5 Gbit/s). Furthermore, a downstream signal #3 uses a bandwidth of 6 Gbit/s.
Moreover, for the upstream, the ONU 93 #1 uses a bandwidth of 2 Gbit/s of the OSU 10 #1 for an upstream signal #1. In addition, the ONU 93 #2 uses a bandwidth of 7 Gbit/s by utilizing the OSU 10 #1 for an upstream signal #2. The ONU 93 #3 uses a bandwidth of 1 Gbit/s by utilizing the OSU 10 #2 for an upstream signal #3.
In this case, since the upstream bandwidth used in the OSU 10 #1 is likely to exceed 9 Gbit/s (due to the upstream signals #1 and #2 described above), the OLT 91 performs a wavelength change instruction such that the upstream signal of the ONU 93 #2 (equivalent to 7 Gbit/s) utilizes the OSU 10 #2.
However, in this case, the downstream signal of the ONU 93 #2 (equivalent to 2.5 Gbit/s) is also changed from the OSU 10 #1 to the OSU 10 #2 at the same time as the switching of the upstream signal.
Consequently, as shown in FIG. 4, the total bandwidth of the downstream signals utilizing the OSU 10 #2 is 11 Gbit/s (the sum of the downstream signal #2 and the downstream signal #3), which exceeds the upper limit of 10 Gbits/s. Therefore, switching cannot be performed as in FIG. 4.
In a wavelength changeover related to the present invention, upstream wavelength and downstream wavelength pairs are assumed to be fixed. Consequently, for example, within the control frame transmitted by the OLT 91 shown in FIG. 5, the wavelength changeover instruction message B also instructs a wavelength changeover for an upstream wavelength and downstream wavelength pair. Here, A, C, D, and E are, for example, a destination address, a source address, a time stamp, and a message identification code of the wavelength changeover instruction message.
Since the control frame shown in FIG. 5 is similar to the frame structure used in the PON architecture described above, existing parts can be applied and cost can be reduced. However, when the upstream wavelength or the downstream wavelength is changed, the other is dragged along with it. Therefore, it is impossible to realize an improvement in the bandwidth utilization efficiency and equitability for both the upstream and the downstream. Further, a bias occurs toward the load distribution of either the upstream or the downstream. When the load distribution is considered, only one load distribution is considered and load distribution of the other is not performed.
Furthermore, at events where there is a concentration of users and the like, if there is a rapid increase in the used bandwidth of only the downstream of a specific ONU 93 resulting from a rapid increase in users under the same ONU 93, then needs such as a change in only the downstream signal wavelength of the ONU 93 to perform bandwidth distribution can be considered. However, in the wavelength tunable WDM/TDM-PON system related to the present invention, since the wavelength of the upstream signal is changed together with the change in the downstream signal, and conversely, since there is a case where the total bandwidth of the ONU 93 using the upstream wavelength exceeds the transmittable bandwidth, it is difficult to realize load distribution of both the upstream and the downstream.